Xiao-Mei Rao

Adenoviruses induce autophagy to promote virus replication and oncolysis.

Adenoviruses with deletion of E1b have been used in clinical trials to treat cancers that are resistant to conventional therapies. The efficacy of viral replication within cancer cells determines the results of oncolytic therapy, which remains poorly understood and requires further improvement. In this report, we show that adenoviruses induce autophagy by increasing the conversion of LC3-I to LC3-II and the formation of the Atg12-Atg5 complex. Inhibition of autophagy with 3-methyladenine (3MA) resulted in a decreased synthesis of adenovirus structural proteins, and thereby a poor viral replication; promotion of autophagy with rapamycin increased adenovirus yield. This study indicates that adenovirus-induced autophagy correlates positively with virus replication and oncolytic cell death, and that autophagy may generate nutrients that can be used for building viral progeny particles. These results further suggest that chemotherapeutic agents that increase cancer cell autophagy may improve the efficacy of oncolytic virotherapy.

Adenovirus E1B55K Region Is Required To Enhance Cyclin E Expression for Efficient Viral DNA Replication

ABSTRACT Adenoviruses (Ads) with E1B55K mutations can selectively replicate in and destroy cancer cells. However, the mechanism of Ad-selective replication in tumor cells is not well characterized. We have shown previously that expression of several cell cycle-regulating genes is markedly affected by the Ad E1b gene in WI-38 human lung fibroblast cells (X. Rao, et al., Virology 350:418-428, 2006). In the current study, we show that the Ad E1B55K region is required to enhance cyclin E expression and that the failure to induce cyclin E overexpression due to E1B55K mutations prevents viral DNA from undergoing efficient replication in WI-38 cells, especially when the cells are arrested in the G0 phase of the cell cycle by serum starvation. In contrast, cyclin E induction is less dependent on the function encoded in the E1B55K region in A549 and other cancer cells that are permissive for replication of E1B55K-mutated viruses, whether the cells are in the S phase or G0 phase. The small interfering RNA that specifically inhibits cyclin E expression partially decreased viral replication. Our study provides evidence suggesting that E1B55K may be involved in cell cycle regulation that is important for efficient viral DNA replication and that cyclin E overexpression in cancer cells may be associated with the oncolytic replication of E1B55K-mutated viruses.

Molecular Basis for Viral Selective Replication in Cancer Cells: Activation of CDK2 by Adenovirus-Induced Cyclin E

Adenoviruses (Ads) with deletion of E1b55K preferentially replicate in cancer cells and have been used in cancer therapies. We have previously shown that Ad E1B55K protein is involved in induction of cyclin E for Ad replication, but this E1B55K function is not required in cancer cells in which deregulation of cyclin E is frequently observed. In this study, we investigated the interaction of cyclin E and CDK2 in Ad-infected cells. Ad infection significantly increased the large form of cyclin E (cyclin EL), promoted cyclin E/CDK2 complex formation and increased CDK2 phosphorylation at the T160 site. Activated CDK2 caused pRb phosphorylation at the S612 site. Repression of CDK2 activity with the chemical inhibitor roscovitine or with specific small interfering RNAs significantly decreased pRb phosphorylation, with concomitant repression of viral replication. Our results suggest that Ad-induced cyclin E activates CDK2 that targets the transcriptional repressor pRb to generate a cellular environment for viral productive replication. This study reveals a new molecular basis for oncolytic replication of E1b-deleted Ads and will aid in the development of new strategies for Ad oncolytic virotherapies.

Adenoviral E1a expression levels affect virus-selective replication in human cancer cells

One of the promising strategies for targeting replication of oncolytic adenovirus in tumor cells is to regulate the expression of essential viral genes such as E1a by using tumor- or tissue-specific promoters that are preferentially active in cancer cells. However, this approach may lead to some degree of viral replication in normal cells other than in cancer cells if the viral gene also expresses in normal cells. In this study, we investigated the effect of E1a expression levels on the virus replication ability in human cells. Three vectors, all with mutated E1B55K, were created, one without any promoter controlling the E1a gene and two vectors with the E1a gene being controlled by either its endogenous promoter or a strong CMV promoter. We observed that the CMV promoter-mediated high levels of E1A expression could increase virus replication, resulting in the titers of the E1B55K-mutated virus being even higher than the wild-type virus in some cancer cells. However, the strong CMV promoter could not always enhance virus replication, such as in cancer cells OE33 and OsACL. The results suggest that whether increased E1A levels would enhance E1B55K-mutated virus replication may be also depended on cellular factors or pathways in cancer cells. We also observed that the virus without any promoter for the E1a gene could still express leaky levels of E1A which can lead to viral replication in normal and cancer cells. Future efforts in the development of transcription-controlled oncolytic adenoviruses should focus on how to completely block E1a expression in normal cells.

Virotherapy targeting cyclin E overexpression in tumors with adenovirus-enhanced cancer-selective promoter

Oncolytic virotherapy can selectively destroy cancer cells and is a potential approach in cancer treatment. A strategy to increase tumor-specific selectivity is to control the expression of a key regulatory viral gene with a tumor-specific promoter. We have previously found that cyclin E expression is augmented in cancer cells after adenovirus (Ad) infection. Thus, the cyclin E promoter that is further activated by Ad in cancer cells may have unique properties for enhancing oncolytic viral replication. We have shown that high levels of viral E1a gene expression are achieved in cancer cells infected with Ad-cycE, in which the endogenous Ad E1a promoter was replaced with the cyclin E promoter. Ad-cycE shows markedly selective oncolytic efficacy in vitro and destroys various types of cancer cells, including those resistant to ONYX-015/dl1520. Furthermore, Ad-cycE shows a strong capacity to repress A549 xenograft tumor growth in nude mice and significantly prolongs survival. This study suggests the potential of Ad-cycE in cancer therapy and indicates the advantages of using promoters that can be upregulated by virus infection in cancer cells in development of oncolytic viruses.Key messagesCyclin E promoter activity is high in cancer cells and enhanced by adenovirus infection.Cyclin E promoter is used to control the E1a gene of a tumor-specific oncolytic adenovirus.Ad-cycE efficiently targets cancer cells and induces oncolysis.Ad-cycE significantly repressed xenograft tumor and prolonged survival.

Adenovirus-Mediated Expression of Truncated E2F-1 Suppresses Tumor Growth In Vitro and In Vivo

Adenovirus (Ad)‐mediated E2F‐1 gene transfer induces apoptosis in cancer cells in vitro and in vivo, but clinical application of E2F‐1 in cancer gene therapy remains controversial because of the oncogenic potential of E2F‐1. This barrier can be circumvented by using the truncated form of the E2F‐1 gene (E2Ftr) (amino acids 1 through 375), which lacks the E2F‐1 transactivation domain and cell cycle‐promoting effects.

E2F-1- and E2Ftr-mediated apoptosis: the role of DREAM and HRK.

E2F‐1‐deleted mutant, ‘truncated E2F’ (E2Ftr, E2F‐1[1–375]), lacking the carboxy‐terminal transactivation domain, was shown to be more potent at inducing cancer cell apoptosis than wild‐type E2F‐1 (wtE2F‐1; full‐length E2F‐1). Mechanisms by which wtE2F‐1 and E2Ftr induce apoptosis, however, are not fully elucidated. Our study demonstrates molecular effects of pro‐apoptotic BH3‐only Bcl‐2 family member Harakiri (Hrk) in wtE2F‐1‐ and E2Ftr‐induced melanoma cell apoptosis. We found that Hrk mRNA and Harakiri (HRK) protein expression was highly up‐regulated in melanoma cells in response to wtE2F‐1 and E2Ftr overexpression. HRK up‐regulation did not require the E2F‐1 transactivation domain. In addition, Hrk gene up‐regulation and HRK protein expression did not require p53 in cancer cells. Hrk knockdown by Hrk siRNA was associated with significantly reduced wtE2F‐1‐ and E2Ftr‐induced apoptosis. We also found that an upstream factor, ‘downstream regulatory element antagonist modulator’ (DREAM), may be involved in HRK‐mediated apoptosis in response to wtE2F‐1 and E2Ftr overexpression. DREAM expression levels increased following wtE2F‐1 and E2Ftr overexpression. Western blotting detected increased DREAM primarily in dimeric form. The homodimerization of DREAM resulting from wtE2F‐1 and E2Ftr overexpression may contribute to the decreased binding activity of DREAM to the 3′‐untranslated region of the Hrk gene as shown by electromobility shift assay. Results showed wtE2F‐1‐ and E2Ftr‐induced apoptosis is partially mediated by HRK. HRK function is regulated in response to DREAM. Our findings contribute to understanding the mechanisms that regulate wtE2F‐1‐ and E2Ftr‐induced apoptosis and provide insights into the further evaluation of how E2Ftr‐induced apoptosis may be used for therapeutic gain.

Developing adenoviral vectors encoding therapeutic genes toxic to host cells: Comparing binary and single-inducible vectors expressing truncated E2F-1

Adenoviral vectors are highly efficient at transferring genes into cells and are broadly used in cancer gene therapy. However, many therapeutic genes are toxic to vector host cells and thus inhibit vector production. The truncated form of E2F-1 (E2Ftr), which lacks the transactivation domain, can significantly induce cancer cell apoptosis, but is also toxic to HEK-293 cells and inhibits adenovirus replication. To overcome this, we have developed binary- and single-vector systems with a modified tetracycline-off inducible promoter to control E2Ftr expression. We compared several vectors and found that the structure of expression cassettes in vectors significantly affects E2Ftr expression. One construct expresses high levels of inducible E2Ftr and efficiently causes apoptotic cancer cell death by activation of caspase-3. The approach developed in this study may be applied in other viral vectors for encoding therapeutic genes that are toxic to their host cells and/or inhibit vector propagation.

Oncolytic adenovirus targeting cyclin E overexpression repressed tumor growth in syngeneic immunocompetent mice

BackgroundClinical trials have indicated that preclinical results obtained with human tumor xenografts in mouse models may overstate the potential of adenovirus (Ad)-mediated oncolytic therapies. We have previously demonstrated that the replication of human Ads depends on cyclin E dysregulation or overexpression in cancer cells. ED-1 cell derived from mouse lung adenocarcinomas triggered by transgenic overexpression of human cyclin E may be applied to investigate the antitumor efficacy of oncolytic Ads.MethodsAd-cycE was used to target cyclin E overexpression in ED-1 cells and repress tumor growth in a syngeneic mouse model for investigation of oncolytic virotherapies.ResultsMurine ED-1 cells were permissive for human Ad replication and Ad-cycE repressed ED-1 tumor growth in immunocompetent FVB mice. ED-1 cells destroyed by oncolytic Ads in tumors were encircled in capsule-like structures, while cells outside the capsules were not infected and survived the treatment.ConclusionAd-cycE can target cyclin E overexpression in cancer cells and repress tumor growth in syngeneic mouse models. The capsule structures formed after Ad intratumoral injection may prevent viral particles from spreading to the entire tumor.

Indole-3-carbinol (I3C) increases apoptosis, represses growth of cancer cells, and enhances adenovirus-mediated oncolysis.

Epidemiological studies suggest that high intake of cruciferous vegetables is associated with a lower risk of cancer. Experiments have shown that indole-3-carbinol (I3C), a naturally occurring compound derived from cruciferous vegetables, exhibits potent anticarcinogenic properties in a wide range of cancers. In this study, we showed that higher doses of I3C (≥400 μM) induced apoptotic cancer cell death and lower doses of I3C (≤200 μM) repressed cancer cell growth concurrently with suppressed expression of cyclin E and its partner CDK2. Notably, we found that pretreatment with low doses of I3C enhanced Ad-mediated oncolysis and cytotoxicity of human carcinoma cells by synergistic upregulation of apoptosis. Thus, the vegetable compound I3C as a dietary supplement may benefit cancer prevention and improve Ad oncolytic therapies.